Maintaining Power and Protection of Electronic Equipment by Choosing the Correct UPS System

Maintaining Power and Protection of Electronic Equipment by Choosing the Correct UPS System

By: Steven Sadler

April 3, 2009

Abstract

Often times the power from the wall socket is neither stable nor uninterrupted. Many abnormalities such as blackouts, spikes, surges, and noise can occur. Under the best conditions, these power interruptions can be an inconvenience. At their worst, they can cause loss of data in computer systems or cause damage to electronic equipment. The primary function of an Uninterruptible Power Supply (UPS) system’s primary function is to provide short-term back-up power allowing time for critical data to be saved and sensitive operating systems to be properly shut down. Another feature of the UPS is that it can provide clean and reliable power to electronic equipment that may be vulnerable to the abnormalities that may occur if your available power source is not reliable.

The basic concept of a UPS is to store energy during normal operation, by charging a battery, or bank of batteries, connected to the UPS and release the energy stored in the batteries, through DC to AC conversion, in the event of a power failure. This application note describes how to correctly select a UPS based on the environment you plan to use the system in, the total wattage of the appliances you plan to connect to the UPS and the desired functions of the UPS system.

Keywords: Uninterruptable Power Supply, UPS, Battery Backup, On-line Power System, True UPS, Off-line Power System, Standby UPS

Background

UPS systems can be categorized as either on-line or off-line (standby). An on-line UPS is constantly engaged, converting all incoming power to DC current that passes though the battery circuit and is then converted back to AC again. Equipment connected to the on-line UPS is electrically isolated from utility power and receives continuous highly regulated power. This design prevents any output power disturbance or gap when there is utility power loss since the battery circuit is always engaged. An on- line UPS can accept as low as 50% normal incoming voltage without drawing battery power, which in turn greatly increases battery life. A figure showing the circuitry of an on-line UPS system is provided in Figure 1 at the end of this document.

Many smaller UPS systems, usually below 5 KVA, are off-line and are typically selected as a cheaper alternative to the on-line system. In an off-line UPS system’s functionality can be described as a switch that routes power directly to your equipment under normal operation. The voltage-sensing switch goes from bypassing the UPS to using the battery when there is a power outage. An off-line UPS does little or no power conditioning and primarily acts as a switch to draw power from the batteries in the result of a power failure. A minor surge in output voltage occurs when it switches from utility power to battery power resulting in a decrease in battery life. Off-line UPS batteries typically last around (2-3) years and are sometimes not replaceable. A figure showing the circuitry of an off-line UPS system is provided in Figure 2 at the end of this document.

In addition to choosing between an on-line and off-line system, you must also decide if you need an indoor and outdoor UPS system. An outdoor UPS system is usually resembles a circuit breaker box that you would see in a home, and is available as a wall/pole mounted UPS or an in-ground UPS. The outdoor UPS circuitry is contained within an enclosure that is designed to protect it from common weather conditions, such as direct sunlight, rain and snow, thus allowing it to be mounted outside. Outdoor UPS systems are typically used for cellular base stations and home security and surveillance systems.

The indoor UPS systems are not equipped with protection from weather and are more portable than the outdoor UPS system. The indoor systems can vary significantly in size depending on the size of the battery; the larger the batter the longer your UPS system will be able to power your appliances while operating on battery backup before shutting down. Indoor UPS systems are more common than outdoor systems and can be used to support a wide range of electronics from a single computer to an entire data center.

Choosing The Correct UPS System

The first question to ask when choosing a UPS system is whether or not the unit will be in a controlled environment. UPS systems are available for various environmental conditions, however you must be certain to select the correct system for your intended usage. If you were to place an indoor UPS in a non-controlled environment, the usable lifetime of the battery and the system itself may be shortened considerably and also backup of the equipment you are protecting, and data stored on that equipment, may also be sacrificed.

Upon determining the environment you wish to place your UPS system in you must also decide the appliances that you wish to connect to the UPS. In order to provide the desired protection you must ensure that the total wattage of the desired appliances does not exceed the capabilities specified by the UPS system. For instance if you choose to provide protection to a computer, with a full processor and hard drive power draw equal to 120 watts, a 17” LCD screen, with a power draw of 20 watts, and a satellite router, with 70 watts of power draw, you would need a UPS system capable of handling up to 210 watts of total load.

After determining the total wattage of your load it is necessary to decide on the desired amount of runtime, the amount of time the UPS battery backup will be able to provide power and protection to the connected appliances total wattage in the event of power loss from your regular power source. The runtime for a UPS is proportional to the total wattage of the load connected to the UPS. The runtime will also vary based on the maximum wattage rating of the UPS. If you are using a UPS that is capable of handling 1500 watts of total load, your runtime will be significantly larger if you only have 750 watts connected to the UPS instead of the full 1500 watts. Each model of UPS will have different specifications for runtime and a chart is typically available on the manufacturers website to help you choose the correct amount of runtime for the load that you plan to connect to the UPS. A typical rule of thumb is to choose a UPS system that is capable of handling double your expected total load wattage, this will usually allow you at least 30 minutes of battery backup from your UPS.

It is important to note that most UPS manufacturers will describe the handling capabilities of the system in both watts and Volt-Amperes (VA). The VA rating is the product of voltage and the amperage that can be drawn on the UPS. Since you are dealing with alternating current, this number is not the same as wattage. For most types of loads, wattage can be expressed as the result of the product of voltage, amps and a "load factor”. The “load factor” is a calculation used to account for the difference in phase between the voltage (Volts) and the current (Amperes) and can be determined by taking the trigonometric cosine of this difference. When the load factor is multiplied by the apparent power, the VA rating given on the UPS, the result is the “true” power, in watts, being used by the load. The VA rating of a power supply negates the load factor and is used mainly to determine heat generation by equipment under use and for sizing wires and circuit breakers. UPS manufacturers design the system for a load factor of between 0.6 and 0.7. So for instance a UPS that is rated at 1000 VA can handle a total load of between 600-700 watts.

Another item worth noting when selecting a UPS is output management software. This software is typically available for larger UPS systems, those rated 750VA and above. Most major UPS manufacturers, such as APC and Tripplite, will provide their customers with a copy of software, upon the purchase of their UPS, that is capable of managing the multi-output ports of the UPS. This software can be very useful and, in most cases, will allow you to shut down each of your available outputs at specific levels of remaining battery life. This software will, again in most cases, allow the user to view and create a report containing the line power voltage, load wattage attached to the UPS, the output voltage of the UPS system and the results of many self tests that are conducted internally by the UPS system.

Conclusion

In this application note I have outlined the basic needs and considerations of a UPS system, including differences in UPS circuitry, environmental protection of the system, and a brief background on how to select the correct for your intended use. Each section of this document includes trade offs to consider for common, or popular, UPS selection and its counterpart. The method of selecting a UPS provided by this application note is intended to give a general background of UPS system and present the customer with the most viable options to consider when selecting a UPS system.

Attached Figures

Figure 1: Schematic Diagram of On-line (“True”) UPS

Figure 2: Schematic Diagram of Off-line (“Standby”) UPS. The solid path shows operation with steady input power. Dashed line is operation when power from the input source has been terminated.

References

All schematic diagrams included in the Attached Figures section were obtained from The PC Guide which can be accessed at http://www.pcguide.com/ref/power/ext/ups/index.htm